If I configure my X in twinview mode (with a LCD monitor connected to my VGA port), all works fine.

If I use dual screen mode, my built-in LCD works, but the attached LCD does not turn on. The screen is logically enabled, as I can move the mouse 'into' the screen, but it is not visible, since the monitor is off.

[Edit: It looks like nvidia-auto-select is defaulting to 1440x900, so if that isn't working for your monitor, you'll have to use something else. 1440x900 is listed as your monitor's preferred mode, so the fact that it's not working is strange.]